Macroalgae, or seaweeds
as they are sometimes called, hold an awkward place in the
reef hobby. If they are not being used for utilitarian purposes
in a refugium, they are either considered as food for herbivores,
or seen as the scourge and plague of otherwise well-run systems.
This series will focus on algal marine plants and attempt
to highlight their more desirable and beautiful attributes.
We will also cover the husbandry and identification for commonly
encountered macroalgae, and review some of the interesting
habitats macroalgae inhabit in the wild.
Whether you love them or hate them, algae are an inevitable
part of reefkeeping. And yes, the word "algae" is
plural. "Alga" is actually the correct term for
a single species and the word "algae" is used to
refer to several species, or a group, of these primitive autotrophs.
It is also fairly common to encounter "macro" as
shorthand for macroalgae in everyday speech and in written
The word "macroalgae" prompts a definition, and
it could be loosely defined as "algae that are visible
to the naked eye." They are not microalgae or phytoplankton,
though several species' spore stages have been misidentified
as species of phytoplankton in the past. It would be helpful
if we could say that macroalgae consist of many cells, and
microalgae of a single cell, but this is not always the case.
Several macroalgae are single-celled, including Acetabularia
(or mermaid's wine glass) and Caulerpa. Additionally,
we could argue that dense blooms of phytoplankton are visible
to the naked eye. So, we begin with a bit of an enigma.
Seaweeds are primitive forms of plants, but because they
lack vascular tissue, they are definitely algae. Macroalgae
species are split into three groups based on their photosynthetic
pigments: Phaeophyta (the browns), Rhodophyta (the reds) and
Chlorophyta (the greens).
Macroalgae fall into two camps among most aquarists: the
dreaded volunteers that grow out of our live rock, and the
useful refugium or decorative species that are deliberately
added to a system. I use the term "volunteers" to
describe macroalgae hitchhikers because they are rarely visible
on freshly purchased rock. They seem to arrive out of nowhere
after a few months or years of dormancy. These volunteers
grow from resting spores, which act as dormant "seeds"
that wait until their growth is triggered by suitable environmental
Several of these volunteers are entirely harmless, though
perhaps unsightly. They do not typically grow fast enough
to be truly noxious, and do not greatly affect water quality
in the system. I have noticed that if these volunteers are
the only major photosynthetic organisms in an aquarium, they
can dominate the system (see photo below). However, macroalgae
belonging to the genera in Table 1 (below) are typically favorable
finds in an aquarium and can be controlled with a little monthly
pruning or with herbivores if they aren't desired in the aquascape.
Neomeris sp. Photo courtesy of Ryngill.
1: Common Volunteer Macroalgae
from Live Rock
or drift algae
Padina sp. (left), Batophora sp. (right).
Photos courtesy of Morten Nordby (left) and Ken Lunde
Other volunteers are more aggressive competitors
for nutrients, light and space, and should be culled from
the tank as soon as possible. These include Dictyota,
Bryopsis (feather algae) and Caulerpa. Two groups
of macroalgae are known by descriptive terms such as red "cotton
candy" algae and red turf algae. Both are menaces in
reef aquariums because they can quickly overgrow corals and
other sessile invertebrates.
Red turf algae, possibly Asparagopsis sp.
Photo courtesy of Marc Levenson (melev).
Dealing with Nuisance Macroalgae
One of the first questions reefkeepers
have when they spy a macroalga in their system is - what,
exactly, is it? While many genera of macroalgae are fairly
easy to identify by appearance alone, most cannot be identified
to species level without an expert education in phycology
(the study of algae). Identification of red algae, in particular,
is a painstaking affair, because there are more species of
red algae than green and brown combined.
If you are adventurous (or curious), several excellent guides
to macroalgae identification are available, including D.S.
Littler's classic, Marine Plants of the Caribbean.
The image search feature at AlgaeBase.org
is also invaluable, as is the image search available through
The drawback to both of these sites is that they require a
few guesses as to the name of the algae before they can begin
searching. It would be impossible to provide a guide to every
macroalga we encounter in our systems, but several galleries
are available online, including Algae
Album - An Algae ID Slideshow and the Algae
Page. Even with these visual aids, a proper identification
is not guaranteed since some species (especially the reds)
can only be accurately identified through the use of a microscope
and access to their other life history stages.
Instead, for some of the nuisance algae, it is more important
to know how to deal with them than to know their exact name.
For Dictyota, Caulerpa and any of the less aggressive
macroalgae, manual removal is sufficient to rein in small
patches of them. It is important to remove all the algae,
including any small pieces that have broken off, or these
may holdfast to a new area of rock and colonize other parts
of the system.
Botryocladia, a red grape algae, grows quickly
under intense lighting in this seagrass aquarium. Photo
courtesy of Scott Fellman.
Many reefkeepers also have success using rabbitfish, triggerfish
and Turbo spp. snails to keep their rock clean. For
hair algae invasions, sea hares are being used successfully
along with lawnmower blennies and emerald crabs. Truly desperate
reefkeepers can employ long spine urchins to try to keep macroalgae
from taking over their tank, or they may opt to remove the
affected rock and clean it of the algae entirely outside the
aquarium. For the best information on controlling Bryopsis,
red turf and "cotton candy" algae, the following
linked threads have invaluable front-line advice: Red
cotton candy algae and Bryopsis.
In your battle against algae, it is
useful to know a little about the basic life cycle of macroalgae.
Most algae follow a fairly predictable life cycle defined
around their reproductive stages. It is not entirely unlike
the life cycle of coral, including a pelagic spore stage and
a dominant benthic stage. In addition, macroalgae can remain
dormant as resting spores attached to the substratum, which
is a departure from the typical life cycle of coral.
Most species of algae grow attached to the substratum from
a spore (analogous to a seed of higher plants) in a vegetative,
growing phase of life. An environmental trigger is typically
required to initiate the reproductive phase of life, which
starts with the formation of gametangia in, or on, the algae.
These structures hold the spores, the gametes of macroalgae.
Spores are released from the parent algae after the parent's
tissues lyse, or break open and die. Macroalgae can spread
in an aquarium, and in the wild, by either growing over substrata,
breaking off and vegetatively colonizing a new area, or settling
onto a new spot as a spore.
Algae happily grow in the vegetative stage of their life
cycle until their environment no longer supports their growth.
Or, in other words, they grow until they run out of food.
In our closed systems, algae take up the nutrients produced
by the aquarium's bioload. The trouble is, it's easy to run
out of the proper ratios of nutrients that algae need to stay
in the growing phase of their life.
Generally, most algae need more nitrogen than phosphorous,
potassium, calcium, magnesium, iron or any other nutrient
with the exception of carbon. Typical aquariums have several
sinks for nitrogen (such as substantial deep sand beds, protein
skimmers and large water changes) and few sinks for other
nutrients. In this situation, the algae grow until their biomass
can no longer be supported by the nutrients available from
the system. When that happens, the sexual stage of the life
cycle is typically triggered. (Note: It can be triggered by
improper photoperiod, inadequate light, or stress from variations
in other tank parameters such as salinity or temperature.)
The best defense against sexual macroalgal events, particularly
in Caulerpa, is a good offense. Regularly trimming
refugium algae is a necessity in reefkeeping. Not only does
this continuously export the nutrients from your tank, it
ensures that the macroalgal biomass does not get too large
for the tank's bioload to support. In addition, iron supplementation
may be useful in aquariums with plentiful sources of nitrogen
and phosphorous, but where refugium macroalgae still look
pale and grow slowly. We truly need more data on iron supplementation
and its effects on macroalgae, fish and other invertebrates.
sp. Photo courtesy of Mike Feely.
Another area which would benefit from a little more investigation
is the potential benefits and risks associated with continuously
lighting a refugium. Many reefkeepers use a twenty-four hour
photoperiod over stands of Caulerpa to prevent sporulation.
Considering the reports from aquarists, this method does work.
We do not know, however, if growth rates are better when using
other photoperiod conditions compared to twenty-four hour
cycles, or if continuous light simply delays an inevitable
Green macroalgae, however, will show warning
signs of impending sporulation, or sexual, events. Both
Halimeda and Caulerpa bleach slightly and show
several hundred tiny green dots on their surface just prior
to releasing their spores (see photo-right). Any algae suddenly
covered in these spots should be quickly removed from the
Sporulation events can be disastrous in reef systems because
the spores are released along with all the cellular contents
of the alga. These contents break down in the aquarium and
can overload the biofilter causing dangerous spikes in ammonia,
nitrite, nitrate and phosphate levels. The break down of all
these organics can also strip the water of oxygen.
Additionally, several genera of macroalgae contain toxic
compounds meant to discourage herbivores from consuming them,
including the toxin caulerpenyne found in Caulerpa.
A few other species contain antibiofouling toxins that discourage
larval invertebrates and spores of other macroalga from settling
on their frond surfaces. While there are no studies that reveal
the toxicity of these compounds to fish and invertebrates
in aquarium environments, it may be prudent to attempt to
clear them from the water rather than risk poisoning tank
If the macroalga succeeds in releasing spores into the tank,
the damage to the system can be minimized by acting quickly.
Use prepared make-up water to do several small water changes
in the system in the days immediately following the event
to help clear decomposing tissue and spores from the water.
Run carbon in filters and pay attention to the protein skimmer
to ensure that the system is adequately filtered. Additionally,
consider additional aeration as insurance for the fish and
Beneficial and Beautiful Macroalgae
Beyond the nuisance and volunteer
algae, some useful and decorative genera are available in
the trade today. Both Sargassum and Halimeda,
which often are volunteers, can be beautiful additions to
an aquarium. Halimeda is a calcareous macroalgae, and
takes up more calcium, magnesium and alkalinity than other
macroalgae, so attention should be given to these levels when
maintaining lots of coral and large stands of Halimeda.
For the refugium, Ulva (sea lettuce), Chaetomorpha
(spaghetti algae) and Gracilaria ("tang heaven")
all make excellent fast-growing nutrient exporters, and double
as food for herbivores and copepods. All three can exist in
a refugium without being tumbled in high circular flow, but
they do best when they spin and all areas of their biomass
are constantly exposed to light and nutrients. All three of
these algae can tolerate both very low and very high light
levels, and can live through hypo- and hypersalinity treatments
in quarantine and hospital tanks. In the case of Chaetomorpha,
its only drawback, is the ability to fragment into short segments
in place of a typical sexual event. Fragments of Chaetomorpha
often break off the main colony and find their way into overflows
and pumps, where they can clog the intakes and cause floods.
To prevent this from occurring, trim Chaetomorpha frequently,
routinely shake it free of any collected detritus, and expand
or turn the mass of strands to allow all the cells access
Ochtodes (blue ball algae), Acanthophora (spiny
seaweed), Botryocladia (one of the common red grape
algae) and Halymenia (dragon's tongue macroalgae) can
also be decorative accents in a reef tank. However, these
are not all the macroalgae that could be suitable in a reef
environment. As more reefkeepers embrace the use of macroalgae
in their systems, we may discover more suitable (and beautiful)
species to propagate. It is also possible to build a marine
planted tank that will sustain only macroalgae. Such a tank
could also maintain most of the Caulerpa species, because
there would be no risk of the algae overgrowing corals.
Husbandry for Desirable Macroalgae
Generally, a planted tank full of
macroalgae can be set up as an attractive refugium. Macroalgae
do well in a range of light and salinity. It would be impossible
to definitively list the light requirements of all the available
species here, but the general range of 60 - 150 PPFD
is more than enough for red, green and brown macroalgae. Like
seagrass, they benefit most from daylight bulbs (regardless
of whether the lights are T5, power compacts or metal halides)
in the 5000 - 10,000K range. A planted tank, like a refugium,
also needs nutrients from the bioload in order to prosper.
Algae can be transplanted into new areas of the tank, or
from sources outside the tank, by attaching small segments
to rubble or live rock. However, many macroalgae do best in
sand and can be placed directly into a (minimum) 1" deep
sand bed (see Table 2 for a list of common macroalgae and
their placement in a system). The fragments can be attached
with coral frag glue, or simply by gently tying them in place
with string or fishing line. Holdfast tissue, which lashes
the algae to the rock, can take weeks to form but will eventually
grow into the new substratum and complete the transplant.
Halimeda sp. (left), Fauchea sp. (center)
and Chaetomorpha crassa (right).
Photos courtesy of Howard Pierce (left), Marc Levenson
(center) and Gary White (right).
Table 2: Placement of Refugium and Decorative
Systems full of macroalgae serve as excellent homes for seahorses,
pipefish, clownfish, gobies, pistol shrimp and other aquarium
fishes and invertebrates. Aquarists who find macroalgae to
be an interesting departure from the more common reef inhabitants
may also merge them with seagrasses, or choose to replicate
macroalgae-dominated habitats. Two particularly interesting
ecosystems rely on macroalgae: the coldwater kelp forests
off the California coast, and the Sargasso Sea in the Atlantic
Ocean. Macroalgae are also important in the turnover of nutrients
within coral and oyster reef ecosystems, and perform this
role in seagrass beds when the true plants are dormant during
The kelp forest is one ecosystem that is rarely replicated
in aquariums due to its need for very large aquaria, extensive
chilling and access to bright light. One of the most successful
live kelp displays is the Kelp Forest exhibit at the Monterey
Bay Aquarium, shots of which stream
live on the Internet. This forest is home to many beautiful
coldwater fish including Garibaldi, rockfish of all colors
and stripes, anemones, sheepsheads, leopard sharks and wolf
eels. A setup recreating this environment would definitely
fall within the realm of advanced aquaria. There is also a
small scale live kelp display (photos below) at the Vancouver
Marine Science Center, in Vancouver, Canada that is more along
the scale of what could be accomplished in a home aquarium
A live kelp display at the Vancouver Marine Science
Center. Photos courtesy of J.C. Delbeek.
A pelagic Sargassum system might be considerably more
within the reach of most aquarists. This macroalgae floats
within a small area defined by currents within an area of
the Atlantic Ocean known as the Sargasso Sea. Many hundreds
of fish species grow up within the floating algae's shelter,
as do newly-hatched sea turtles. The algae here literally
form life rafts and oases within the open ocean's desert-like
conditions. A system with floating Sargassum, and a
bit of underlying rock and sand, would certainly make a dramatic
stage for Sargassum-associated filefish, anglers and
seahorses. Members of the Sargassum genus can be found
in other areas of the world, including as rafts within the
Pacific and Indian Oceans.
Finally, one of the easiest macroalgae dominated tanks to
create may be a volunteer alga garden. One of my personal
favorite methods of acquiring macroalga is to find live rock
covered in small segments that are typically cast aside by
other reefkeepers. These make excellent seed rocks for your
evolving marine planted tank, and it can be interesting to
watch the alga grow and take shape in your system. If your
live rock doesn't sprout enough biodiversity for your taste,
fragments of macroalgae ship well from fellow reefkeepers
and are often easy to finesse from your favorite reef shop.
Pictured on the left is a macroalgae garden including
Acanthophora, various Caulerpa and Botryocladia.
Halymenia (right photo), dragon's tongue algae,
grows quickly under intense lighting in this seagrass
aquarium. Left photo courtesy of Bruce Watts;
right photo courtesy of John Boiger.
Other Uses of Macroalgae
Humans use macroalgae outside aquariums
in several ways, including consuming it whole as nori or wrapped
around sushi. Derivatives of macroalgae are routinely used
as thickening agents in food. The polysaccharide carrageenan
can be found in many products including ice cream, vegan Jell-O,
soy milk, shampoos, fire-fighting foam, several toothpastes
and even in beer, where it is employed to remove residual
haze. These uses arise from several species, principally Chondrus
crispus, also known as Irish moss, a red macroalgae typical
along coldwater coasts.
Interestingly, researchers from the National Cancer Institute
recently published findings that carrageenan had properties
that decreased human papillomavirus infections of cells in
laboratory screenings (Buck et al., 2006). The current
research boom in biotechnology may increase the use of algae
in our society. Instead of new pharmaceuticals from the rainforest,
we may be obtaining them from
In fact, algae already speed along research on college campuses
and in private laboratories worldwide, as the polysaccharides
agar and agarose. These are also derived from red macroalgae,
and are used to form the gel used to create Petri dishes that
researchers use to work with bacteria, fungi, animal cells
and phytoplankton. They are also used in a slew of experiments
to study genes and DNA.
Closer to home, the recent resurgence of holistic and homeopathic
medicine has renewed interest in the use of macroalgal extracts.
Lining supermarket shelves, many of the lotions, creams and
moisturizers that claim to firm skin also contain extracts
from macroalgae. And, in perhaps one of the more bizarre and
interesting uses of macroalgae, vegans and other people interested
in organic clothing have turned to Seacell, which combines
seaweed and wood pulp to produce a fabric that claims to nourish
skin as you wear the garment. Clearly, macroalgae aren't just
in your aquarium.
Finally, no article on macroalgae
would be complete without a warning to the reefkeeping community
about invasive species of algae. Many of us are aware of Caulerpa
taxifolia, the so-called killer alga that invaded the
Mediterranean in 1984 and later turned up on the coast near
San Diego, California in 2000. Nine species of Caulerpa
are banned in California, including C. taxifolia, C.
racemosa, C. cupressoides, C. mexicana,
C. verticillata, C. sertularoides, C. ashmeadii,
C. floridana and C. scalpelliformis. Generally
speaking, if its morphology at all resembles a feather or
a grape, it is not welcome in California.
Photo courtesy of Marc Levenson (melev).
As responsible aquarists, we need to remember that our fish
are not the only creatures in our care that can damage an
ecosystem. It is very simple to add bleach to old water during
maintenance to kill organisms from the tank, and to kill any
harvested macroalgae before pouring the tank water down the
drain. Or, if you're opposed to bleach, freezing the alga,
or leaving it out to dry completely, is also effective.
Currently, the marine aquarium trade has grudgingly accepted
macroalgae as refugium exporters. However, macroalgae may
still have some useful secrets to yield to marine aquarists.
Macroalgae already do so much for our society and our oceans
that they may certainly give some benefits to reefkeeping
- as exporters, as decorative accents and as a new habitat
in marine aquaria.
I would like to thank John Boiger,
Scott Fellman, Mike Feely, Marc Levenson, Bruce Watts and
J. C. Delbeek for the use of their photographs and their comments
during the writing of this article. I would also like to thank
those that participate in the Marine
Plants and Macroalgae forum on Reef Central for their
contributions to this fledgling branch of the hobby.